The invention relates to a method for cleaning stone and metal surfaces and an apparatus for carrying out said method. In particular the invention relates to a method and an apparatus for cleaning surfaces of stone and metal contaminated and corroded by atmospheric influences, for example facades of this type or stone and metal monuments.
The stone surfaces cleaned according to the invention may be both artificial stone surfaces such as concrete surfaces or also natural stone surfaces such as limestone surfaces or granite surfaces.
Because of pronounced air pollution the cleaning of such surfaces like the surfaces of monuments or statues cast usually from bronze is becoming of increasing importance. As a rule, when cleaning such surfaces only the dirt and soil layer should be removed. Usually, the metal layer therebelow corroded by atmospheric pollutants is to be retained.
The important point is to remove as little material as possible. In particular, the stone or metal material disposed therebelow must not be removed. In the case of bronze figures not even the natural patina, if any is present, should be removed.
A cleaning method having the features of the preamble of claim 1 is known from U.S. Pat. No. 3,427,763. In this known cleaning method a pressurized water flow generated by means of a water pressure between 100 and 900 bar in a mixing chamber sucks the blast material in from a passage opening laterally into the mixing chamber, said blast material having a granulation between 0.01 and about 3 mm and consisting of sand, quartz, corundum, flue dust and the like. The water jet acts as water jet pump and in this manner draws in the blast material particles.
The intention is that because the blast material particles are carried by a water jet and thrown against the surface to be cleaned that the blast material particles do not simply strike against said surface to be cleaned. On the contrary, at least mainly, they are to be entrained by the sprayed-on water, slide along the surface and in this manner clean the surface.
An essential disadvantage of this known method is that too much of the material to be worked is removed. Accordingly, the known method is used primarily for cleaning coarse parts, such as castings and the like, and in addition also as separating cutting method in which the water jet charged with blast material saws a gap through the workpiece to be severed. Thus, the known method is not suitable for cleaning valuable objects, for example historical buildings, monuments and the like. It is not possible in practice to conduct the known method so that only the upper layer to be removed is in fact removed and the material therebelow is not impaired.
The object of the invention is to further develop the known method so that the cleaning of the object surfaces can take place on the one hand more rapidly but on the other in such a manner that removal of parts of the object surface is avoided or is only negligible.
The cleaning is perfect, i.e. no dirt or soil residues are left, and also there is no discolouring or other disadvantageous influencing of the object surface, providing the method is correctly applied.
According to the invention this problem is solved in that the jet apart from the water and the blast material contains a high proportion of air which by volume is several times the proportion of water, that the jet rotates about its axis and that the jet under the influence of the air contained therein under pressure a the start of the jet and of the rotation expands greatly laterally. Thus, the jet emerging from the tool for carrying out the method has substantially the form of a cone in which the angle between the cone axis and one generatrix of the cone surface as a rule is between 20.degree. and 40.degree..
Due to the fact that the jet contains a high proportion of air it assumes the character of a water-in-air dispersion. The air contained therein under pressure at the start of the jet expands when the jet emerges into the atmosphere and effects the conical fanning of the jet towards all sides. The rotation of the air-blast material-water mixture acts in the same sense. This rotation also uniformly expands the jet radially towards all sides. On the path from the generation point, usually a nozzle, to the surface to be cleaned the cross-section of the jet thus increases approximately proportionally to the square of the distance from the origin of the jet. The velocity component of the jet in the direction of the jet axis, i.e. in the direction of the cone axis, decreases however relatively little because the increase of the flow cross-section of the jet does not take place as in the prior art, if present, by velocity reduction but by expansion of the air contained in the jet. In addition, any velocity reduction in the jet which might occur is compensated by the expansion of the air because this expansion acts of course not only radially outwardly but also in the jet propagation direction.
It has been found that when working with a cleaning agent jet of the type explained not only metal surfaces, in particular bronze surfaces, but also natural and artificial stone surfaces can be easily and safely cleaned. The method according to the invention is particularly suitable for sharp-edged blast material, such as glass powder. Suprisingly, the surface to be cleaned is not unduly removed. On the contrary, the removal remains astonishingly low although perfect removal of the soil layers is effected. Applicants assume that this is due to the fact that the method according to the invention responds to an unusually great extent to different hardnesses in the surface regions of the object to be cleaned. This means that the soft dirt layers are rapidly removed whereas the stone material is hardly attacked by the blast material particles sliding over its surface and no doubt partially executing there circular movements. Consequently, the worker cleaning an object's surface with the apparatus generating a jet according to the invention no longer runs the risk of inadmissibly attacking the object's surface by allowing the jet to continue to act even for a short time on an adequately cleaned surface. This makes it possible to further clean stubbornly soiled areas without having to take excessive care as regards adjacent already cleaned regions.
An essential criterion of the method according to the invention resides in that said method can easily be adapted to the hardness of the surface to be worked and cleaned.
If for example a limestone or marble facade is to be cleaned the water pressure and thus also the pressure of the air supplying the blast material will be made low whilst for cleaning hard surfaces, for example granite surfaces or hard bronze surfaces, the pressure may be made relatively high.
A further advantage of the invention compared with the prior art is that a considerable velocity component parallel to the surface to be worked is imparted to the jet material particles not only by the rotation and expansion of the jet prior to impinging on said surface but in addition the removal effect of the blast material in the invention is distributed over a far greater area than was the case with the narrow jets according to the prior art. This also contributes to a particularly mild removing effect. Surprisingly, this only gentle removing effect of the cleaning jet according to the invention is adequate to obtain a rapid perfect cleaning by removal of soil layers.
It is considered essential in the invention to admix an adequately large amount of air. It is obvious that the admixture of smaller amounts of air can only lead to a slight expansion of an approximately cylindrical jet. Accordingly, the air is admixed in such a high proportion that the air contained in the jet is many times by volume the amount of water therein. By volume, the proportion of air in the jet is advantageously about 200 times to 1200 times the water proportion, the air proportion by volume of course greatly increasing in the jet propagation direction due to the expansion of the jet.
By weight, the air proportion remains substantially constant. It is advantageously 0.5 to 3 times the water proportion, and the air proportion should be greater the greater the water pressure. Air porportions from 0.7 to 1.5 have proved suitable.
Accordingly, a cleaning jet according to the invention does not have the relatively dark colour of the water charged with the blast or abrasive material. Such a jet rather has a white appearance.
The jet according to the invention is preferably formed in that in a mixing chamber a mixture under considerable excess pressure of sharp-edged blast material, water and air is generated, said mixture set in rotation about an axis and the rotating mixture sprayed out along the axis. In this manner in the mixing chamber a good mixing of air, blast material and water can be achieved. However, in the mixing chamber a relatively high pressure is maintained which is also utilized to eject the jet from the mixing chamber unless this ejection is effected by retaining the kinetic energy of the water jet entering the mixing chamber.
Because the air in the mixing chamber is still at a pressure only slightly below the pressure at which it was introduced into said mixing chamber, its volume remains correspondingly small. Immediately after emergence of the blast-material water-air mixture from the mixing chamber into the ambient atmosphere the air can expand and thus radially expand the jet.
Preferably, the method is carried out in such a manner that a pressurized water jet is injected into the mixing chamber at the side thereof opposite the exit nozzle in the direction towards said nozzle and that a pressurized air flow entraining blast material is directed from the side obliquely forwardly against the water jet in such a manner that the jet centre axis of the air jet and the jet centre axis of the water jet extend in spaced relationship from each other. Due to the eccentric impingement of the flows on each other a considerable rotation is generated in the mixing chamber.
Fundamentally, the rotation can also be differently generated, for example by injecting the water tangentially into a mixing chamber. However, the rotation is preferably generated in the manner explained above. This has the essential advantage that the rotation generated is not excessive because if it is the blast material particles would be entrained too much into the outer edge regions of the jet generated. However, in the preferred embodiment of the invention in which the mixing chamber tapers conically towards the exit nozzle this is counteracted by the fact that in the mixing chamber as well blast material particles rotating near the periphery thereof on their way to the nozzle are given a movement component directed radially inwardly towards the mixing chamber axis. In this manner the blast material particles are very uniformly distributed in the conically expanded jet so that the cleaning effect of said jet occurs over the entire impingement cross-section thereof on the surface to be cleaned.
Preferred parameters for performing the method according to the invention can include the water pressure prior to entry into the chamber being about 70 to 130 bar, the excess pressure of air with with the blast material is supplied is about 3 to 8%, preferrably about 5% of the water pressure, and that a ratio of 1 kg of blast material to 3 to 50 kg of water is supplied, preferrably 1 kg to 6 kg of water.
The blast or abrasive material is preferably ground glass power which is correspondingly sharp-edged and has a granulation between 0 and 1 mm, preferably between 0 and 0.5 mm.
The invention also relates to an apparatus for carrying out the method.
With such an apparatus execution of the method according to the invention is relatively simple. To obtain the desired jet structure firstly only the water supply is set with the desired pressure, for example 50 bar. The blast-material air supply is then connected and the pressure of the air entraining the blast material increased until the initially rod-shaped jet leaving the exit nozzle becomes white and assumes the form of a cone. The jet has now the structure used according to the invention which has the essential advantages explained above as regards the cleaning even of sensitive surfaces.
It is essential in the invention to use a blast material which is sharp-edged. The importance of sharp-edges is shown by the fact that the reuse of glass powder used once as blast material leads to a comparatively poorer cleaning effect and, with correspondingly more intense action, to greater removal of the object surface to be cleaned. Accordingly, glass powder is used as blast material preferably only once.
Fundamentally, of course, other materials such as ground quartz or ground flint can be used. This is however more complicated. The same applies to the use of corundum or other commerically usual abrasive powders.
The best results were achieved when the blast material had grains of different magnitude up to 1 mm, preferably up to 0.5 mm. The use of grains of different magnitude leads to better cleaning action than that of grains of identical magnitude. Preferably, the grain size of the blast material is distributed in accordance with a normal distribution curve over the range from 0 up to the maximum size. With regard to the term normal distribution curve reference is made to the book "Introduction to Granulation Measurement Techniques" by Bartel (Springer Publications, Berlin, Gottingen, Heidelberg, 1964), pages 13 and 14.
The path of the normal distribution curve is preferably such that about half (by weight) of all the grains have a size between one third and two thirds of the maximum size. With the preferred granulation of the sharp-edged irregularly shaped grains of the blast material half thereof should thus have a granulation between 0.17 mm and 0.33 mm.